Ring laser gyroscopes (referred as laser gyros for brevity) use an optical ring resonator which defines a unique sensitive axis about which inertial rotation is to be monitored. A gain medium within the resonator is used to generate and sustain one of the Gaussian, or so-called fundamental, modes of electromagnetic energy, the fundamental mode having components traveling around the resonator in clockwise and counter-clockwise directions. Rotation of the resonator about its sensitive axis results in a frequency splitting between the clockwise and counterclockwise components. In order to avoid frequency locking that occurs at low rotation rates, laser gyros normally use a biasing scheme such that, for all expected rotation rates, the frequencies of the counter-propagating components are separated by an amount greater than the lock-in frequency range. In an optical biasing scheme typically used for four-frequency laser gyros, a direction-dependent (nonreciprocal) polarization rotation means, such as a Faraday rotator, and a direction-independent (reciprocal) polarization rotation means produce a resonator which sustains a fundamental mode having four components, each with a different frequency. One pair of frequency components has a first polarization sense, and another pair of frequency components has a second polarization sense orthogonal to the first, each one of the two pairs having the two frequency components propagating in opposite directions around the resonator. The rotation rate is measured by determining the difference in the frequencies of each of the pairs of oppositely traveling mode components to produce a pair of frequency difference signals. An output signal is then produced to provide a measure of rotation rate, such output signal being produced by taking the difference between the pair of frequency difference signals. Such laser gyro, is described in U.S. Pat. No. 3,741,657, issued June 26, 1973 to K. Andringa, assigned to the present assignee.
The frequencies of the resonating mode components are a function of the pathlength, and a change of pathlength, due to, for example, a temperature change, will cause the mode components to shift in frequency and thereby drift under the gain curve. Since the gain medium provides an index of refraction which is a function of frequency and since the frequency of the propagating waves, absent any pathlength control, changes, each of the two pairs of frequency difference signals changes differently independent of actual rotation rate. This effect is sometimes called gain medium induced frequency dispersion, and it gives rise to an apparent rotation rate which is, in part, a function of the pathlength. Then, even for moderate accuracy applications, the pathlength of the resonator for such prior laser gyros must be carefully controlled to produce a stable output signal.